This invention was the subject matter of Document Disclosure Program Registration Numbers 309646, 310441, and 312981 which were filed in the United States Patent and Trademark Office on May 18, 1992, May 26, 1992 and Jul. 15, 1992 respectively.
It has been well known that steering the front wheels and rear wheels in opposite direction, i.e. the reverse-phase direction steering, in slow vehicle speed can achieve small turning radius capability. However, the rear portion of the vehicle can be projected outward during forward driving under the reverse-phase direction steering.
It also has been demonstrated that steering the front wheels and rear wheels in the same direction, i.e. the coincidence-phase direction steering, in high vehicle speed can promote the stability of the vehicle and decrease the possible lateral slippage of the wheels.
During direction changing at high vehicle speed, the centrifugal force can produce lateral skidding of the wheels or spinning of the vehicle. Although the coincidence-phase direction steering can reduce or prevent lateral skidding of wheels, it creates the problems of understeering and decreased yawing, often in untimely situations. A certain degree of yawing, i.e. vehicle orientation change, is necessary for a vehicle to properly change its direction.
In U.S. Pat. No. 5,103,925 Imaseki et al. disclosed a rear wheel steering system which steered rear wheels in coincidence-phase direction for rear-wheel drive cars, and in reverse-phase direction for front-wheel drive cars, to prevent driven wheel slippage during cornering.
In the U.S. Pat. No. 4,979,116, Dec. 18, 1990, Takahashi described an invention which further modified the above reference value of the rear wheel steered angle. At high or medium vehicle speed, the rear wheel steering speed reference value was controlled to reduce the coincidence-phase direction steering speed of the rear wheels at low front wheel steering speed; and to increase the coincidence-phase direction steering speed of the rear wheels at high front wheel steering speed. By this design, Takahashi was able to delay the starting of the coincidence-phase steering of the rear wheels during slow speed of the steering wheel. When the steering wheel was turned abruptly at medium or high vehicle speed, the rear wheels were quickly steered in the coincidence-phase. This invention partially solved the problems of steering sensitivity deterioration, i.e. understeeing and decreased yawing, during coincidence-phase direction steering of the rear wheels. However, the problems were solved only partially. Besides, this type of design may sometimes incorrectly interfere with the driver's genuine intention and correct steering effort. When the driver makes an abrupt turning of the steering wheel, often the driver has some good reasons to do that. The unsolicited change of regular steering to coincidence-phase direction steering very likely may interfere with the driver's intention and effort.
In the U.S. Pat. No. 5,048,629, Sep. 17, 1991, Abe et al. described an invention in which the rear wheels were steered by a variable steering ratio which was predetermined in matching relation to vehicle speed.
In the U.S. Pat. No. 5,076,597, Dec. 31, 1991, Korekan et al. disclosed an invention in which the rear wheel steering was determined by three different ranges of steering wheel rotation. In the first angular range of steering wheel rotation, the rear wheels were steered in coincidence-phase. In the second angular range of the steering wheel rotation, the rear wheels were held in the neutral direction. When the steering wheel was rotated above the second angular range, the rear wheels were steered in the reverse-phase.
In the U.S. Pat. No. 5,083,627, Jan. 28, 1992, Kawamoto et al. disclosed a rear wheel steering mechanism in which the rear wheel steering was based on both the front wheel steered angle and the vehicle speed.
In all of the above prior arts, the rear wheels were steered indirectly, depending on the following factors: the vehicle speed, and/or steering wheel angle, and/or steering wheel rotating speed. Therefore, in all of these types of designs, the rear wheel steered angle does not have a constant relationship with the rotated angle of the steering wheel. Because of this, the yawing rate is variable in all of the prior arts.
Yawing means the turning by angular motion about the vertical axis. For automobiles; it refers to the variation in vehicle orientation. Yawing is necessary in car driving when changing direction. For the ordinary two-front-wheel steering vehicles, the yawing rate is constant, said yawing rate means the relationship between degree of steering wheel rotation and degree of vehicle orientation changing. Constant and predictable yawing rate is the easiest for the driver to control, and is most comfortable with the best steering feedback.
For four-wheel steering vehicle, during reverse-phase direction steering, the yawing effect is due to the sum of front wheels' direction changing plus the rear wheels' direction changing. For coincidence-phase direction steering, the yawing effect is related to the front wheels' direction changing minus the rear wheels' direction changing. Since in all of the prior arts, the rear wheels' steered angle did not have a constant relationship with the steering wheel rotated angle, the vehicle yawing rate is quite variable. The steering feedback became irregular, unpredictable, often delayed and sometime hastened. This often produce a feeling of loss of steering control. The present invention will solve this problem by providing constant and instant yawing rate, except when modified designs are adopted to improve cornering stability or to prevent wheel skidding.
There was a dilemma not totally solved by any of the prior arts. As pointed out by Abe et al. in U.S. Pat. No. 5,048,629, Sep. 17, 1991, the reverse-phase steering at slow vehicle speed has the advantage of very small turning radius. However, the rear portion of the vehicle can be projected to the outside under reverse-phase steering. To prevent this problem, some prior arts used designs to delay the rear wheel steering until after the vehicle has traveled a predetermined distance, or until the the vehicle has exceeded a certain speed, etc. Another prior art, by Trefz et al. U.S. Pat. No. 5,048,835, Sep. 17, 1991, disclosed a dwell period between the movement of the front and that of the rear steering assemblies. However, with these types of designs, the benefit of small turning radius capability will be significantly degraded.
In the present invention, with the novel concept of choices of multiple steering modes, the above dilemma and problems can be solved. In this invention, the reverse-phase steering is under a separate steering mode, Steering Mode Four, which will be used only when the driver needs to make very small turning radius at slow vehicle speed. It is true that under Steering Mode Four, the rear portion of the vehicle will still be projected outward during forward driving; and the front portion will be projected outward during backward driving. However, since this phenomenon occurs only during this special circumstance, i.e. the driver specifically intends to make very small turning radius at very slow vehicle speed, the driver can easily pay attention to the surroundings so that the vehicle does not collide with anything on its side. It is well known that when driving an ordinary vehicle backward with turning, the front portion of the vehicle will be projected outward, actually twice as much as under the reverse-phase steering, i.e. the Steering Mode Four. Since all drivers can manage this problem easily at slow vehicle speed, the Steering Mode Four driving should have no difficulty at all.
As discussed by Abe et al. in U.S Pat. No. 5.048,629, Sep. 17, 1991, the steering ratio of the front wheels and rear wheels was usually set, by many prior arts, such that the front and rear wheels were steered in the reverse-phase in a low vehicle speed range, and in the coincidence-phase in a high vehicle speed range. These types of designs mixed the reverse-phase steering and the coincidence-phase steering into one steering mode. Korekan et al, in their U.S. Pat. No. 5,076,597, Dec. 31, 1991, also mixed the coincidence-phase steering, regular front wheel steering, and the reverse-phase steering all into one steering mode, depending on three angular ranges of steering wheel rotation. However, it is well known that each of the above three different steering methods has its own advantages and disadvantages and that the advantages and disadvantages of one steering method often contradict those of the other steering methods. Even with the various designs of the prior arts, none of them were able to solve these contradictions completely. It is one of the objectives of this present invention to separate each steering method from others by the multiple steering mode system. This will successfully solve the above problems.
To summarize, the problems of the prior arts, which will be solved in this invention, include the following:
1. Rear wheel steerage is dependent on other factors. The rear wheel steered angle can be decreased, increased, delayed or hastened. The yawing rate varies too much. PA0 2. Driver often has poor steering feedback and feeling of loss of steering control, because of the variable and unpredictable yawing rate. PA0 3. Poor utilization of the coincidence-phase direction steering, resulting in untimely side-effects, including understeering and decreased yawing rate, and interference with the driver's intention. The vehicle orientation may become improper and may require further steering maneuvering to correct it. PA0 4. Poor utilization of reverse-phase direction steering, resulting in significant loss of part of its benefit of small turning radius capability. PA0 5. Lack of choices of multiple steering modes. A single steering mode had lots of difficulties to accommodate a plurality of different steering methods. A single steering mode can not totally avoid the contradictions between different steering methods.